Spatial light modulators are devices for controlling the spatial distribution of the intensity of light (i.e., near-IR, visible, and UV). Such devices, which can be used in processing data, are capable of spatially modulating a collimated coherent or incoherent beam of light with, for example, input data which is to be processed. The devices are appropriately coupled to optical data processing systems into which the data-modulated light beam is supplied at a rate commensurate with the processing system's potential throughput; the optical processing system utilizes parallel processing without the limitations normally imposed by serial manipulation of the data.
Optical computing structures hold the promise of providing highly parallel, high speed computational architectures. Such structures will be capable of image recognition and processing tasks unapproachable with conventional computing. A major impediment to practical optical computers is the absence of suitable optical processing elements.
Many optical computational schemes rely on programmable optical masks or spatial light modulators.
Implementations of optical computers for "conventional" as well as "neural network" computing schemes are limited by the speed, pixel density, programmability, and cost of these SLMs.
Existing optically addressed spatial light modulators (SLMs) can be characterized as either multielement or single element systems. In the multielement systems, separate elements are required to act as photosensors and modulating materials. These systems tend to be complex, expensive, and slow. Existing single element materials can act as SLMs based on various principles. These include photoconductive electro-optic materials (such as bismuth silicon oxide, BSO), photorefractive materials (such as barium titanate or gallium arsenide), materials which undergo electroabsorption (Franz-Keldysh effect, such as gallium arsenide), or thermally sensitive materials (such as vanadium dioxide or smectic liquid crystals). Table I (adapted from A. D. Fisher and J. N. Lee, SPIE, Vol. 634, p. 352 (1986)) lists the device characteristics of several of these "state of the art" spatial light modulators (lp=line pairs).
TABLE I ______________________________________ Performance Characteristics of Existing Spatial Light Modulators Switching Typical Resolution energy Response Type Material (lp/mm) (.mu.J/cm.sup.2) time (msec) ______________________________________ Pockels BSO 10 5 0.1 Volume BaTiO.sub.3 1500 1 to 10.sup.4 &lt;30 Hologram Electro- GaAs 16 .times. 1 -- &lt;10.sup.-6 absorption pxls. Thermal liq. cryst. 40 10.sup.4 5 .times. 10.sup.-3 Multi- liq. cryst. 30 6 1 to 10 element ______________________________________
Liquid rrystal SLMs, developed primarily for display applications, are relatively slow, low density devices. A great need exists for fast, high density programmable (write many times) SLMs for optical computing applications.
It is clear from this Table that the existing materials do not possess the desired combination of high resolution, low switching energy, and fast response.